Pregnant women quit smoking because it’s bad for the baby – and better for the pocketbook, a new study indicates.

Researchers found that hiking cigarette prices increases the number of expecting moms who will kick the habit.

The result is a better birth weight for their babies, according to the study, whose results are published in the Journal of Public Economics.

”Twenty to 25 percent of women quit smoking when they find out they’re pregnant. This study shows that price hikes seem to put some more women over the top,” said study author William Evans of the University of Maryland.

”The good news is that the drop in smoking rates by pregnant women in response to higher cigarette prices results in an increase in the birth weight of the babies born to these women.

”The bad news is that the pregnant women who continue to smoke don’t smoke any less because of the higher prices.”

The study zeroed in on smoking rates among 10 million women who gave birth before and after state tobacco-tax hikes.

Every 10 percent increase in cigarette prices results in a 5 percent drop in smoking rates among pregnant women, the researchers reported.

Factoring in women who cross state borders to buy cheaper cigarettes – and taking into account anti-smoking laws – didn’t significantly change the results, they said.

Based on their findings, the researchers predicted that a $1.10 increase in cigarette prices would reduce the number of pregnant smokers by 30 percent.

That, in turn, would reduce the rate of low birth weights by about 5 percent.

”Several medical experts have estimated that cigarette smoking accounts for up to 20 percent of all low birth weight in the country,” Evans said.

Price hikes would also reduce the number of smokers in the population at large – but pregnant women are more sensitive to the added costs, he said.

The study was funded by the Substance Abuse Policy Research Program of The Robert Wood Johnson Foundation.